The present invention relates to a lithium-ion rechargeable battery, comprising a negative electrode that comprises graphite as an active material and carbon nano-tubes as a conductive additive.
As electronic equipments are increasingly made portable and cordless, small-sized, lightweight lithium-ion rechargeable batteries having a high energy density are drawing attention as power sources for driving such electronic equipments. Rocking chair type lithium-ion rechargeable batteries have already come into practice and rapidly become common. A typical lithium-ion rechargeable battery comprises a positive electrode comprising a lithium-containing transition metal composite oxide as an active material, a negative electrode comprising a carbon material as an active material, a separator, and a non-aqueous electrolyte.
In particular, graphite with high crystallinity has recently been in the mainstream of carbon materials to be used as the negative electrode active material. Graphite is advantageous in the following aspects: (1) electronic conductivity of graphite is high and discharge performance with a large current is excellent; (2) an electrical potential change of graphite during discharge is small and graphite is thus suitable for such applications as discharging with a constant electric power; and (3) graphite has a large true density and is obtained as particles with a large bulk density and is advantageous for increasing an energy density of a battery.
Graphite for a negative electrode of lithium-ion rechargeable batteries which are currently under development and consideration can be classified into two types: natural graphite particles and artificial graphite particles.
As the natural graphite particles, massive natural graphite can be cited which is prepared by transforming flake particles into massive (spherical) particles in a pulverizing step of raw ore or other steps (Japanese Laid-Open Patent Publication No. Hei 11-263612).
Further, the following can be cited as the artificial graphite particles: a material prepared by graphitizing some sort of coke, and a material prepared by graphitizing granulated particles made of a coke and various sorts of pitch etc. (Japanese Laid-Open Patent Publication No. Hei 6-310142, Japanese Laid-Open Patent Publication No. Hei 10-188959), and special artificial graphite particles utilizing mesophase carbon (sort of liquid crystal) produced by heating pitch or tar.
As the special artificial graphite particles, mainly used ones may include: (1) a material prepared by carbonizing and graphitizing mesophase spherules having been separated and extracted from mesophase pitch (graphitized MCMB) (Japanese Laid-Open Patent Publication No. Hei 4-190555, Japanese Laid-Open Patent Publication No. Hei 5-307958); (2) a material prepared such that mesophase pitch in a fused state, which generates in the polymerization-growth process of mesophase spherules, is spun into fiber, and after the obtained fiber is made infusible through surface oxidation, it is carbonized and then cut and pulverized to be graphitized (graphitized milled MCF (mesophase pitch based carbon fiber)) (Japanese Laid-Open Patent Publication No. Hei 9-63584); and (3) a material prepared such that pulverized particles of bulk mesophase pitch with low fusibility, having generated in the polymerization-growth process of mesophase spherules, is carbonized and graphitized (graphitized bulk mesophase) (Japanese Laid-Open Patent Publication No. Hei 9-251855, Japanese Patent No. 3309701 Specification, Japanese Laid-Open Patent Publication No. Hei 9-259886).
As for the natural graphite particles, a reversible capacity close on the theoretical capacity of graphite of 372 mAh/g has been obtained. In this regard, there have been accumulated techniques for adjusting a form of particles to make them suitable for high density filling (Japanese Laid-Open Patent Publication No. Hei 11-54123), for example, in response to a recent demand for a higher energy density of lithium-ion rechargeable batteries. Further actively considered has been reduction in irreversible capacity that occurs through electrolyte decomoposition on the surface of graphite particles at the initial stage of charging, by covering an edge face exposed to the graphite particle surface with amorphous carbon classified as graphitizable carbon.
In the case of the artificial graphite, on the other hand, a reversible capacity close to the theoretical capacity of graphite cannot be obtained at the present time. Since the artificial graphite has a reversible capacity smaller than that of the natural graphite, studies are underway on increasing purities of cokes, pitch and tar as raw materials, and also on raising a graphitization level of particles to improve a reversible capacity by making conditions for graphitization appropriate according to materials, by adding a catalyst species for promoting graphitization to the material, or by some other methods. It should be noted that, in such artificial graphite, the ratio of the graphite edge face exposed to the particle surface is small and the irreversible capacity at the initial stage of charging is generally smaller than that of the natural graphite.
In actual production of a negative electrode of a lithium-ion rechargeable battery, such graphite species as described above is adjusted to have an average particle size in the range of 5 μm to tens of μm before being used. There are some cases where one sort of the above graphite species is singly used as an active material, and there is another case where two sorts or more of the above graphite species are mixed and then used as an active material. In general, an aqueous paste or an organic paste is prepared and then applied onto a negative electrode current collector, such as a copper foil. The applied paste is dried so that a negative electrode material mixture layer is formed to serve as a negative electrode plate with the current collector. The negative electrode material mixture layer is rolled by pressure to have a prescribed thickness (density). Subsequently, the negative electrode plate is cut, processed, and then subjected to such a process as lead-welding to the exposed part of the current collector.
The aqueous paste is prepared by adding adequate amounts of water, SBR (styrene-butadiene copolymer rubber) or the like as a binder, and CMC (carboxymethyl cellulose) or the like as a thickener, respectively, in active material particles comprising graphite. Further, the organic paste is prepared by adding adequate amounts of PVDF (polyvinylidene fluoride) or the like as a binder/thickener and NMP (N-methyl-2-pyrrolidone) or the like as a dispersion medium, respectively, in active material particles comprising graphite.
It is often the case that the upper limit of the density of the negative electrode material mixture layer is set to about 1.7 g/cm3, to prevent crush (collapse) of active material particles, which occurs during the rolling by pressure, and to prevent drop or separation of particles from the current collector. In such a manner that the aforementioned negative electrode, a positive electrode rolled by pressure in the same manner as with the negative electrode, and a polyolefin-made microporous separator having an adequate porosity and mechanical strength, are combined to assemble a lithium-ion rechargeable battery, it has become possible to obtain a volume energy density exceeding 350 Wh/L
Meanwhile, there has hitherto been a problem with a lithium-ion rechargeable battery using a negative electrode comprising such graphite as above described: a problem of capacity deterioration in the course of charge/discharge cycles. Herein, deterioration in cycle life characteristic attributable to a negative electrode comprising graphite can be understood from the following aspect:
With repetition of intercalating/deintercalating lithium ions to and from the spacing between graphite layers (expansion and shrinkage of graphite particles) during charge/discharge cycles, the following problem may arise.
First, graphite particles crack or collapse and a newly formed edge face of the graphite is exposed to an electrolyte, causing consumption by decomposition of the electrolyte to increase internal resistance of a battery (deterioration mode 1). Further, the graphite particles float from the current collector and a negative electrode material mixture swells. As a result, performance of collecting current between the graphite particles becomes deficient, and some of the graphite particles are left isolated within a material mixture layer and cannot contribute to charge/discharge reactions. This can also be one of the major causes of the capacity deterioration. (deterioration mode 2).
Due to swelling (expansion) of an electrode, an electrode plate assembly comprising a positive electrode and a negative electrode deforms or breaks to cause the capacity deterioration (deterioration mode 3). Further, a gas produced by the electrolyte decomposition causes the battery internal pressure to increase, leading to the capacity deterioration according to deformation of a case (deterioration mode 4).
Herein, in response to the deterioration caused by the electrolyte decomposition on the graphite particles as in the deterioration modes 1 and 4, there have recently been conducted intensive studies including a study on application of a negative electrode protection additive, such as vinylene carbonate (VC), into an electrolyte (Japanese Laid-Open Patent Publication No. Hei 8-45545, Japanese Laid-Open Patent Publication No. 2002-25612) [measure 1]. VC reacts preferentially with graphite in the negative electrode at the first stage of charging/discharging, to form a protective film on the graphite particles so as to prevent the decomposition reaction from occurring between the graphite and the electrolyte due to cycles.
In response to the deterioration mode 2, there have hitherto been proposed methods [measure 2] for reducing the isolated particles within the material mixture layer by adding a conductive additive for current collection to graphite as a main active material. The methods include: addition of carbon fiber to graphitized MCMB or the like (Japanese Laid-Open Patent Publication No. Hei 4-237971, Japanese Laid-Open Patent Publication No. Hei 4-155776), addition of carbon black or the like, which is non-graphitized carbon having a chain structure to graphitized MCMB or the like (Japanese Laid-Open Patent Publication No. Hei 4-332465), addition of flake graphite to graphitized MCMB or the like (Japanese Laid-Open Patent Publication No. 2000-138061), and addition of a low-crystalline coke to natural graphite (Japanese Laid-Open Patent Publication No. Hei 8-264181).
In response to the deterioration mode 3, it is effective to use a negative electrode material with a low level of swelling, namely particles with relatively high isotropy of a graphite structure. Among those described above, preferably used ones may include the material obtained by graphitizing granulated particles made of a coke and various sorts of pitch, or the like, graphitized MCMB, and graphitized milled MCF. In using graphite particles with large anisotropy, such as natural graphite particles represented by flake graphite, it is essential to mechanically transform flake particles into spherical ones, or perform other treatments (Japanese Laid-Open Patent Publication No. Hei 11-263612) [measure 3].
It should be noted that application of carbon nano-tubes, in place of graphite, as a main active material of a negative electrode of a lithium-ion rechargeable battery, has already been under review (Japanese Laid-Open Patent Publication No. Hei 5-159804, Japanese Laid-Open Patent Publication No. Hei 7-14573). However, there has been found no example of studies on the use of carbon nano-tubes as a conductive additive in terms of improving a cycle life characteristic.
As thus described, although various measures (the measures 1 to 3) have been taken toward lithium-ion rechargeable batteries with the aim of improving cycle life characteristics thereof, it is hard to say that cycle life characteristics of lithium-ion rechargeable batteries have reached a sufficient level. In particular, the method of adding a conductive additive to graphite, as shown in the measure 2, leaves much to be improved.